In order to be able to filter large amounts of air in for instance, air conditioning installations, it is known to use folded filters with a very great depth. These filters are sometimes called bag-filters since they can be considered as a group of bags located adjacent one another. In addition to achieving a large area for the filter material in this way, the filter bags can be folded for transport and storage, which in particular results in transport savings since the filter in the shape when used is particularly bulky. As a result of the large area of the separate bags these however have a tendency to inflate so that the triangular cross section shape desired for a good flow is easily lost. In order to counteract this it is known to sew fast threads cross-wise through the bag. While this has no negative effect on the foldability of the filters, the sewing of threads in the filter material may however easily lead to damage either during sewing or later when the filter is subjected to a load. Primarily these damages may result in leakages, which is why glue usually is applied as a sealant. In other words the number of steps required for the fabrication of these filters and in particular the manual effort involved is comparatively large. Furthermore, the use of glue makes the material non-homogeneous which is a disadvantage when it comes to recycling.
In view of the above problems of filters of this kind the object of the invention is to achieve a filter and a fabrication method that can be executed simpler, faster, and with a more complete homogeneity of material.